The field of this invention pertains to materials for electrical contacts. Specifically, electrical contact materials comprising gold combined with small amounts of dispersed refractory and rare earth oxides is the subject matter of this invention.
Pure gold, or high karat gold alloys, are extensively used for connector applications. These materials have limited use as make and break contacts because of poor resistance to wear and a tendency to stick or weld at fairly low current values. To meet the requirements of specific applications, such as sensitive relays, instruments, computers, key switches, slip-rings and brushes, radio frequency tuners, and telecommunication applications, a number of special high-content gold alloys have been developed. In most applications, these alloys are used only up to a maximum current level of approximately 0.5 to 2 amperes when long life and low contact resistance are required.
The outstanding electrical contact characteristic of gold is its immunity to the formation of high resistance films of oxides, sulfides, or organic materials. Other advantageous properties are its good electrical conductivity (approx. 73% IACS, International Annealed Copper Standard), low yield point, and low modulus of elasticity, all of which combine to assure a low and stable contact resistance. These properties make it suitable for connectors and for electrical contacts operating at low contact pressure and low current, such as up to 100 to 300 milliamperes. However, the low hardness and low recrystallization temperature of gold lead to excessive mechanical wear, a high tendency for welding and galling, and excessive material loss due to arc erosion when used for make and break contacts at higher current values.
For these reasons most commercial gold contact materials are high-content gold alloys containing other noble metals (e.g., platinum or silver) or base metals (e.g., copper or nickel), in order to provide substantially greater resistance to mechanical wear and to arc erosion. It should be noted that there are two principal factors which limit the amount of alloy additions that can be made to produce these wrought alloys. The first is that the electrical conductivity decreases rapidly with alloy additions; many of these alloys have a conductivity in the range of 4-12% IACS, which limits the current carrying capacity of these materials. The second factor is that alloy additions to gold must be limited in order to retain its outstanding characteristics, such as its resistance to the formation of oxides and films. Thus noble metals and silver additions are generally limited to about 30-40 atomic percent, and base metal additions to 14-18 Kt alloys (58-75 weight percent Au).
Commercial gold alloys developed for specific applications are based on the best compromise of contact erosion, welding tendency, and low contact resistance (noise), and in most applications are generally limited to a maximum current value of 0.5 to 2.0 amperes when long life (10.sup.6 to 10.sup.8 operations) is required. Failure or end-of-life in these applications is generally reached because of (1) formation of a spike and crater erosion pattern, which may lead to bridging the contact gap and result in an interlocking type of weld; (2) actual welding of the contacts, which is considerably enhanced by excessive erosion, or the formation of small molten globules or whiskers on the contact surface and edges; or (3) the development of high and variable contact resistance which results in excessive electrical noise.
It is known that the elevated temperature strength and hardness of metals can be significantly increased by the addition of a finely dispersed stable oxide phase. Theories of dispersion strengthening are well developed and good agreement of experimental data with theory has been observed. However, the effect of these oxides on the important electrical contact characteristics, such as arc erosion, weld tendency, and change in contact resistance, is little known and less understood. Silver-cadmium oxide is a contact material of this type, consisting of CdO dispersed in a silver matrix. However, silver-cadmium oxide is in a special category, since Cdo is not a stable oxide such as is required for dispersion strengthening, particularly at elevated temperature. In silver-cadmium oxide contacts the CdO phase is volatile and decomposes (at approximately 1700.degree.-1750.degree. F) during arcing; this feature gives this material its unique arc-quenching characteristics, especially when used in heavy current applications of 10-50 amps and higher. It should also be noted that these materials contain a fairly high oxide content, usually 10-15%. Even when present in small amounts there is no appreciable strengthening effect of CdO on silver, and above 15% CdO these alloys are too brittle to be fabricated by conventional methods. One of the outstanding properties contributed by CdO to silver is that it decreases the amount of material lost by arc erosion.
It is therefore a general object of this invention to provide an electrical contact made from material which results from the small addition of refractory and rare earth oxides which have been found to have a desirable strengthening effect on gold.
It is a specific object of this invention to provide an electrical contact made from material comprising small additions of refractory and rare earth oxides to gold which reduce the erosion and welding properties of gold contacts while achieving a low and stable surface contact resistance.